We propose to examine the chemical nature of chromosomes which results in G-banding. Since chromosome banding is reversible, it is suggested that reversible protein conformational changes affect the DNA's accessibility to Giemsa and thus inhibits staining in interbands. These studies will examine the protein alterations occurring in chromosomes during each step of the G-banding procedure. Toward this goal large quantities of chromosomes will be isolated and (1) fixed, (2) treated with tyrpsin or hot salt, stained with methylene blue, a component of Giemsa and (3) destained. The chemical composition and protein gel profiles of the chromosomes from each of the three indicated steps of the banding procedure will be determined as will protein removal, DNA accessibility, degree of protein denaturation and degree of ionic binding to DNA. DNA accessibility will be measured with the probes methylene blue, staphylococcal nuclease, dansylated and partially hydrolyzed polylysines of various DNA affinities and psoralin. Protein denaturation will be measured by the availability of interior disulfide groups to reduction. Ionic binding of protein to DNA will be indirectly measured from the availability of epsilon-amino groups of lysine to react with dansyl chloride. These studies therefore examine a number of physical parameters of chromosomal DNA and protein which should permit a greater understanding of the structural significance of a visible chromosome band. Such understanding is important for our conceptualization of the chromomere, chromosome structure and organization and for the development of improved cytogenetic banding techniques.